<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <title>Postfix Architecture Overview </title> <meta http-equiv="Content-Type" content="text/html; charset=us-ascii"> </head> <body> <h1> <img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix Architecture Overview </h1> <hr> <h2> Introduction </h2> <p> This document presents an overview of the Postfix architecture, and provides pointers to descriptions of every Postfix command or server program. The text gives the general context in which each command or server program is used, and provides pointers to documents with specific usage examples and background information. </p> <p> Topics covered by this document: </p> <ul> <li> <a href="#receiving"> How Postfix receives mail </a> <li> <a href="#delivering"> How Postfix delivers mail </a> <li> <a href="#behind"> Postfix behind the scenes </a> <li> <a href="#commands"> Postfix support commands </a> </ul> <h2><a name="receiving"> How Postfix receives mail </a> </h2> <p> When a message enters the Postfix mail system, the first stop on the inside is the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>. The figure below shows the main processes that are involved with new mail. Names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues. </p> <blockquote> <table> <tr> <td colspan="4"> </td> <td bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a> </td> </tr> <tr> <td> Network </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td> </td> <td rowspan="2" align="center"> <table> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> </table> </td> </tr> <tr> <td colspan="3"> </td> <td> <tt> \ </tt> </td> </tr> <tr> <td> Network </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="qmqpd.8.html">qmqpd(8)</a> </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="cleanup.8.html">cleanup(8)</a> </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="QSHAPE_README.html#incoming_queue"> incoming </a> </td> </tr> <tr> <td colspan="3"> </td> <td> <tt> / </tt> </td> </tr> <tr> <td colspan="2"> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="pickup.8.html">pickup(8)</a> </td> <td> <tt> <- </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="QSHAPE_README.html#maildrop_queue"> maildrop </a> </td> </tr> <tr> <td colspan="4" align="center"> </td> <td align="center"> ^<br> <tt> | </tt> </td> </tr> <tr> <td> Local </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="sendmail.1.html">sendmail(1)</a> </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="postdrop.1.html">postdrop(1)</a> </td> </tr> </table> </blockquote> <ul> <li> <p> Network mail enters Postfix via the <a href="smtpd.8.html">smtpd(8)</a> or <a href="qmqpd.8.html">qmqpd(8)</a> servers. These servers remove the SMTP or QMQP protocol encapsulation, enforce some sanity checks to protect Postfix, and give the sender, recipients and message content to the <a href="cleanup.8.html">cleanup(8)</a> server. The <a href="smtpd.8.html">smtpd(8)</a> server can be configured to block unwanted mail, as described in the <a href="SMTPD_ACCESS_README.html">SMTPD_ACCESS_README</a> document. </p> <li> <p> Local submissions are received with the Postfix <a href="sendmail.1.html">sendmail(1)</a> compatibility command, and are queued in the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> by the privileged <a href="postdrop.1.html">postdrop(1)</a> command. This arrangement even works while the Postfix mail system is not running. The local <a href="pickup.8.html">pickup(8)</a> server picks up local submissions, enforces some sanity checks to protect Postfix, and gives the sender, recipients and message content to the <a href="cleanup.8.html">cleanup(8)</a> server. </p> <li> <p> Mail from internal sources is given directly to the <a href="cleanup.8.html">cleanup(8)</a> server. These sources are not shown in the figure, and include: mail that is forwarded by the <a href="local.8.html">local(8)</a> delivery agent (see next section), messages that are returned to the sender by the <a href="bounce.8.html">bounce(8)</a> server (see second-next section), and postmaster notifications about problems with Postfix. </p> <li> <p> The <a href="cleanup.8.html">cleanup(8)</a> server implements the final processing stage before mail is queued. It adds missing From: and other message headers, and transforms addresses as described in the <a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a> document. Optionally, the <a href="cleanup.8.html">cleanup(8)</a> server can be configured to do light-weight content inspection with regular expressions as described in the <a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a> document. The <a href="cleanup.8.html">cleanup(8)</a> server places the result as a single file into the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>, and notifies the queue manager (see next section) of the arrival of new mail. </p> <li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server rewrites addresses to the standard "user@fully.qualified.domain" form, as described in the <a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a> document. Postfix currently does not implement a rewriting language, but a lot can be done via table lookups and, if need be, regular expressions. </p> </ul> <h2> <a name="delivering"> How Postfix delivers mail </a> </h2> <p> Once a message has reached the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> the next step is to deliver it. The figure shows the main components of the Postfix mail delivery apparatus. Names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues. </p> <blockquote> <table> <tr> <td rowspan="2" colspan="4"> </td> <td rowspan="2" bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a> </td> <td> </td> <td bgcolor="#f0f0ff" align="center"> <a href="smtp.8.html">smtp(8)</a> </td> <td> <tt> -> </tt> </td> <td> Network </td> </tr> <tr> <td align="right"> <tt> / </tt> </td> </tr> <tr> <td rowspan="2" colspan="4"> </td> <td rowspan="2" align="center"> <table> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> </table> </td> <td align="right"> <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="lmtp.8.html">lmtp(8)</a> </td> <td> <tt> -> </tt> </td> <td> Network </td> </tr> <tr> <td align="left"> <tt> / </tt> </td> </tr> <tr> <td bgcolor="#f0f0ff" align="center"> <a href="QSHAPE_README.html#incoming_queue"> incoming </a> </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="QSHAPE_README.html#active_queue"> active </a> </td> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td align="right"> <tt> --- </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="local.8.html">local(8)</a> </td> <td> <tt> -> </tt> </td> <td> File, command </td> </tr> <tr> <td rowspan="2" colspan="2"> </td> <td rowspan="2" align="center"> <table> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> </table> </td> <td rowspan="2" colspan="2"> </td> <td align="left"> <tt> \ </tt> </td> </tr> <tr> <td align="right"> <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="virtual.8.html">virtual(8)</a> </td> <td> <tt> -> </tt> </td> <td> File </td> </tr> <tr> <td colspan="2"> </td> <td bgcolor="#f0f0ff" align="center"> <a href="QSHAPE_README.html#deferred_queue"> deferred </a> </td> <td colspan="2"> </td> <td align="right"> <tt> \ </tt> </td> </tr> <tr> <td colspan="6"> <td bgcolor="#f0f0ff" align="center"> <a href="pipe.8.html">pipe(8)</a> </td> <td> <tt> -> </tt> </td> <td> Command </td> </tr> </table> </blockquote> <ul> <li> <p> The queue manager (the <a href="qmgr.8.html">qmgr(8)</a> server process in the figure) is the heart of Postfix mail delivery. It contacts the <a href="smtp.8.html">smtp(8)</a>, <a href="lmtp.8.html">lmtp(8)</a>, <a href="local.8.html">local(8)</a>, <a href="virtual.8.html">virtual(8)</a>, <a href="pipe.8.html">pipe(8)</a>, <a href="discard.8.html">discard(8)</a> or <a href="error.8.html">error(8)</a> delivery agents, and sends a delivery request for one or more recipient addresses. The <a href="discard.8.html">discard(8)</a> and <a href="error.8.html">error(8)</a> delivery agents are special: they discard or bounce all mail, and are not shown in the figure above. </p> <p> The queue manager maintains a small <a href="QSHAPE_README.html#active_queue">active queue</a> with the messages that it has opened for delivery. The <a href="QSHAPE_README.html#active_queue">active queue</a> acts as a limited window on potentially large <a href="QSHAPE_README.html#incoming_queue">incoming</a> or <a href="QSHAPE_README.html#deferred_queue">deferred queues</a>. The limited <a href="QSHAPE_README.html#active_queue">active queue</a> prevents the queue manager from running out of memory under heavy load. </p> <p> The queue manager maintains a separate <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> for mail that cannot be delivered, so that a large mail backlog will not slow down normal queue accesses. The queue manager's strategy for delayed mail delivery attempts is described in the <a href="QSHAPE_README.html">QSHAPE_README</a> and <a href="TUNING_README.html">TUNING_README</a> documents. </p> <li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server resolves each recipient address according to its local or remote address class, as defined in the <a href="ADDRESS_CLASS_README.html">ADDRESS_CLASS_README</a> document. Additional routing information can be specified with the optional <a href="transport.5.html">transport(5)</a> table. The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server optionally queries the <a href="relocated.5.html">relocated(5)</a> table for recipients whose address has changed; mail for such recipients is returned to the sender with an explanation. </p> <li> <p> The <a href="smtp.8.html">smtp(8)</a> client looks up a list of mail exchangers for the destination host, sorts the list by preference, and tries each server in turn until it finds a server that responds. It then encapsulates the sender, recipient and message content as required by the SMTP protocol; this includes conversion of 8-bit MIME to 7-bit encoding. </p> <li> <p> The <a href="lmtp.8.html">lmtp(8)</a> client speaks a protocol similar to SMTP that is optimized for delivery to mailbox servers such as Cyrus. The advantage of this setup is that one Postfix machine can feed multiple mailbox servers over LMTP. The opposite is true as well: one mailbox server can be fed over LMTP by multiple Postfix machines. </p> <li> <p> The <a href="local.8.html">local(8)</a> delivery agent understands UNIX-style mailboxes, qmail-compatible maildir files, Sendmail-style system-wide <a href="aliases.5.html">aliases(5)</a> databases, and Sendmail-style per-user .forward files. Multiple local delivery agents can be run in parallel, but parallel delivery to the same user is usually limited. </p> <p> The <a href="local.8.html">local(8)</a> delivery agent has hooks for alternative forms of local delivery: you can configure it to deliver to mailbox files in user home directories, you can configure it to delegate mailbox delivery to an external command such as procmail, or you can delegate delivery to a different Postfix delivery agent. </p> <li> <p> The <a href="virtual.8.html">virtual(8)</a> delivery agent is a bare-bones delivery agent that delivers to UNIX-style mailbox or qmail-style maildir files only. This delivery agent can deliver mail for multiple domains, which makes it especially suitable for hosting lots of small domains on a single machine. This is described in the <a href="VIRTUAL_README.html">VIRTUAL_README</a> document. </p> <li> <p> The <a href="pipe.8.html">pipe(8)</a> mailer is the outbound interface to other mail processing systems (the Postfix <a href="sendmail.1.html">sendmail(1)</a> command being the inbound interface). The interface is UNIX compatible: it provides information on the command line and on the standard input stream, and expects a process exit status code as defined in <sysexits.h>. Examples of delivery via the <a href="pipe.8.html">pipe(8)</a> mailer are in the <a href="MAILDROP_README.html">MAILDROP_README</a> and <a href="UUCP_README.html">UUCP_README</a> documents. </ul> <h2> <a name="behind"> Postfix behind the scenes </a> </h2> <p> The previous sections gave an overview of how Postfix server processes send and receive mail. These server processes rely on other server processes that do things behind the scenes. The text below attempts to visualize each service in its own context. As before, names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues. </p> <ul> <li> <p> The resident <a href="master.8.html">master(8)</a> server is the supervisor that keeps an eye on the well-being of the Postfix mail system. It is typically started at system boot time with the "postfix start" command, and keeps running until the system goes down. The <a href="master.8.html">master(8)</a> server is responsible for starting Postfix server processes to receive and deliver mail, and for restarting servers that terminate prematurely because of some problem. The <a href="master.8.html">master(8)</a> server is also responsible for enforcing the server process count limits as specified in the <a href="master.5.html"><b>master.cf</b></a> configuration file. The picture below gives the program hierarchy when Postfix is started up. Only some of the mail handling daemon processes are shown. </p> <table> <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> <a href="postfix.1.html">postfix(1)</a> </td> </tr> <tr> <td colspan="2"> </td> <td align="center"> |<br> |</td> </tr> <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> postfix-script(1) </td> </tr> <tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center"> |<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td> </tr> </table> </td> </tr> <tr> <td align="center" bgcolor="#f0f0ff"> <a href="postsuper.1.html">postsuper(1)</a> </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> <a href="master.8.html">master(8)</a> </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> <a href="postlog.1.html">postlog(1)</a> </td> </tr> <tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center"> |<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td> </tr> </table> </td> </tr> <tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> <a href="local.8.html">local(8)</a> </td> </tr> </table> <li> <p> The <a href="anvil.8.html">anvil(8)</a> server implements client connection and request rate limiting for all <a href="smtpd.8.html">smtpd(8)</a> servers. The <a href="TUNING_README.html">TUNING_README</a> document provides guidance for dealing with mis-behaving SMTP clients. The <a href="anvil.8.html">anvil(8)</a> service is available in Postfix version 2.2 and later. </p> <table> <tr> <td> Network </td> <td> <tt> -> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a><br><br> </td> <td> <tt> <-> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="anvil.8.html">anvil(8)</a><br><br> </td> </tr> </table> <li> <p> The <a href="bounce.8.html">bounce(8)</a>, <a href="defer.8.html">defer(8)</a> and <a href="trace.8.html">trace(8)</a> services each maintain their own queue directory trees with per-message logfiles. Postfix uses this information when sending "failed", "delayed" or "success" delivery status notifications to the sender. </p> <p> The <a href="trace.8.html">trace(8)</a> service also implements support for the Postfix "sendmail -bv" and "sendmail -v" commands which produce reports about how Postfix delivers mail, and is available with Postfix version 2.1 and later. See <a href="DEBUG_README.html#trace_mail"> DEBUG_README </a> for examples. </p> <table> <tr> <td align="center" bgcolor="#f0f0ff"> <a href="cleanup.8.html">cleanup(8)</a> </td> <td valign="middle"> <tt> -> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a><br> Postfix<br> queue </td> <td valign="middle"> <tt> -> </tt> </td> <td align="center" bgcolor="#f0f0ff"> Delivery<br> agents</td> </tr> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td> </td> <td align="center"> <tt> |<br> v </tt> </td> <td> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> <tr> <td align="center"> (Non-)<br> delivery<br> notice </td> <td valign="middle"> <tt> <- </tt> </td> <td align="center" bgcolor="#f0f0ff"> <a href="bounce.8.html">bounce(8)</a><br> <a href="defer.8.html">defer(8)</a><br> <a href="trace.8.html">trace(8)</a> </td> <td valign="middle"> <tt> <- </tt> </td> <td align="center"> Queue id,<br> recipient,<br> status</td> </tr> <tr> <td colspan="2"> </td> <td align="center"> <table> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> </table> </td> </tr> <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> Per- <br> message<br> logfiles </td> </tr> </table> <li> <p> The <a href="flush.8.html">flush(8)</a> servers maintain per-destination logs and implement both ETRN and "sendmail -qRdestination", as described in the <a href="ETRN_README.html">ETRN_README</a> document. This moves selected queue files from the <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> back to the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> and requests their delivery. The <a href="flush.8.html">flush(8)</a> service is available with Postfix version 1.0 and later. </p> <table> <tr> <td colspan="4"> </td> <td align="center" bgcolor="#f0f0ff"> <a href="QSHAPE_README.html#incoming_queue"> incoming </a><br>^ <br><a href="QSHAPE_README.html#deferred_queue"> deferred </a> </td> </tr> <tr> <td colspan="4"> </td> <td align="center"> ^<br> |</td> </tr> <tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a><br> <a href="sendmail.1.html">sendmail(1)</a><br> <a href="postqueue.1.html">postqueue(1)</a> </td> <td> <tt> - </tt> </td> <td align="center"> Destination<br> to flush</td> <td> <tt> -> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <a href="flush.8.html">flush(8)</a> </td> <td> <tt> <- </tt> </td> <td align="center"> Deferred<br> destination,<br> queue id </td> <td> <tt> - </tt> </td> <td align="center" bgcolor="#f0f0ff"> Delivery<br> agents,<br> <a href="qmgr.8.html">qmgr(8)</a> </td> </tr> <tr> <td colspan="4"> </td> <td align="center"> <table> <tr> <td align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> </td> </tr> </table> </td> </tr> <tr> <td colspan="4"> </td> <td align="center"> Per-dest-<br> ination<br> logs </td> </tr> </table> <li> <p> The <a href="proxymap.8.html">proxymap(8)</a> servers provide read-only and read-write table lookup service to Postfix processes. This overcomes chroot restrictions, reduces the number of open lookup tables by sharing one open table among multiple processes, and implements single-updater tables. </p> <li> <p> The <a href="scache.8.html">scache(8)</a> server maintains the connection cache for the Postfix <a href="smtp.8.html">smtp(8)</a> client. When connection caching is enabled for selected destinations, the <a href="smtp.8.html">smtp(8)</a> client does not disconnect immediately after a mail transaction, but gives the connection to the connection cache server which keeps the connection open for a limited amount of time. The <a href="smtp.8.html">smtp(8)</a> client continues with some other mail delivery request. Meanwhile, any <a href="smtp.8.html">smtp(8)</a> process can ask the <a href="scache.8.html">scache(8)</a> server for that cached connection and reuse it for mail delivery. As a safety measure, Postfix limits the number of times that a connection may be reused. </p> <p> When delivering mail to a destination with multiple mail servers, connection caching can help to skip over a non-responding server, and thus dramatically speed up delivery. SMTP connection caching is available in Postfix version 2.2 and later. More information about this feature is in the <a href="CONNECTION_CACHE_README.html">CONNECTION_CACHE_README</a> document. </p> <table> <tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center" colspan="3" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td colspan="2"> <tt> --> </tt> Internet </td> </tr> <tr> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> </td> <td align="center" rowspan="3"> </td> <td align="center" rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> <td> </td> </tr> <tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center" colspan="2" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td align="left"> <tt> --> </tt> Internet </td> </tr> <tr> <td colspan="3"> </td> <td align="center"><tt>^<br>|</tt></td> <td> </td> </tr> <tr> <td colspan="3"> </td> <td align="center" colspan="3" bgcolor="#f0f0ff"> <a href="scache.8.html">scache(8)</a> </td> </tr> </table> <li> <p> The <a href="showq.8.html">showq(8)</a> servers list the Postfix queue status. This is the queue listing service that does the work for the <a href="mailq.1.html">mailq(1)</a> and <a href="postqueue.1.html">postqueue(1)</a> commands. </p> <table> <tr> <td> Output </td> <td> <tt> <- </tt> </td> <td align="center" bgcolor="#f0f0ff"> <a href="mailq.1.html">mailq(1)</a><br> <a href="postqueue.1.html"> post-<br>queue(1) </a> <br> </td> <td> <tt> <- </tt> </td> <td align="center" valign="middle" bgcolor="#f0f0ff"> <a href="showq.8.html">showq(8)</a> </td> <td> <tt> <- </tt></td> <td align="center" valign="middle" bgcolor="#f0f0ff"> Postfix<br> queue </td> </tr> </table> <li> <p> The <a href="spawn.8.html">spawn(8)</a> servers run non-Postfix commands on request, with the client connected via socket or FIFO to the command's standard input, output and error streams. You can find examples of its use in the <a href="SMTPD_POLICY_README.html">SMTPD_POLICY_README</a> document. </p> <li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server runs when TLS (Transport Layer Security, formerly known as SSL) is turned on in the Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a> server. This process has two duties: </p> <ul> <li> <p> Maintain the pseudo-random number generator (PRNG) that is used to seed the TLS engines in Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a> server processes. The state of this PRNG is periodically saved to a file, and is read when <a href="tlsmgr.8.html">tlsmgr(8)</a> starts up. </p> <li> <p> Maintain the optional Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a> server caches with TLS session keys. Saved keys can improve performance by reducing the amount of computation at the start of a TLS session. </p> </ul> <p> TLS support is available in Postfix version 2.2 and later. Information about the Postfix TLS implementation is in the <a href="TLS_README.html">TLS_README</a> document. </p> <table> <tr> <td>Network<tt>-> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a> <br> </td> <td colspan="2"> <tt> <---seed---<br><br><-session-> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="tlsmgr.8.html">tlsmgr(8)</a> <br> </td> <td colspan="3"> <tt> ---seed---><br> <br><-session-> </tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtp.8.html">smtp(8)</a> <br> </td> <td> <tt> -></tt>Network </td> </tr> <tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td> </td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center"> |<br> |</td> <td align="left"> <table> <tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td> </tr> </table> </td> <td colspan="3"> </td> </tr> <tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> smtpd<br> session<br> cache </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td align="center" bgcolor="#f0f0ff"> smtp<br> session<br> cache </td> <td colspan="2"> </td> </tr> </table> <li> <p> The <a href="verify.8.html">verify(8)</a> server verifies that a sender or recipient address is deliverable before the <a href="smtpd.8.html">smtpd(8)</a> server accepts it. The <a href="verify.8.html">verify(8)</a> server queries a cache with address verification results. If a result is not found, the <a href="verify.8.html">verify(8)</a> server injects a probe message into the Postfix queue and processes the status update from a delivery agent or queue manager. This process is described in the <a href="ADDRESS_VERIFICATION_README.html">ADDRESS_VERIFICATION_README</a> document. The <a href="verify.8.html">verify(8)</a> service is available with Postfix version 2.1 and later. </p> <table> <tr> <td rowspan="2" colspan="5" align="center" valign="middle"> </td> <td rowspan="3" align="center" valign="bottom"> <tt> -> </tt> </td> <td rowspan="3" align="center" valign="middle"> probe<br> message </td> <td rowspan="3" align="center" valign="middle"> <tt> -> </tt> </td> <td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle"> Postfix<br> mail<br> queue </td> </tr> <tr> <td> </td> </tr> <tr> <td rowspan="3" align="center" valign="middle"> Network </td> <td rowspan="3" align="center" valign="middle"> <tt> -> </tt> </td> <td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td rowspan="3" align="center" valign="middle"> <tt> <-> </tt> </td> <td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle"> <a href="verify.8.html">verify(8)</a> </td> </tr> <tr> <td rowspan="1" colspan="3"> </td> <td rowspan="1" align="center" valign="middle"> <tt> |</tt><br> <tt> v</tt> </td> </tr> <tr> <td rowspan="3" align="center" valign="top"> <tt> <- </tt> </td> <td rowspan="3" align="center" valign="middle"> probe<br> status </td> <td rowspan="3" align="center" valign="middle"> <tt> <- </tt> </td> <td rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle"> Postfix<br> delivery<br> agents </td> <td rowspan="3" align="left" valign="middle"> <tt>-></tt> Local<br> <tt>-></tt> Network</td> </tr> <tr> <td rowspan="3" colspan="4" align="center" valign="middle"> </td> <td rowspan="3" align="center" valign="middle"> <tt> ^</tt><br> <tt> |</tt><br> <tt> v</tt> </td> </tr> <tr> <td> </td> </tr> <tr> <td colspan="4"> </td> </tr> <tr> <td colspan="4" align="center" valign="middle"> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> Address<br> verification<br> cache </td> </tr> </table> <li> <p> The <a href="postscreen.8.html">postscreen(8)</a> server can be put "in front" of Postfix <a href="smtpd.8.html">smtpd(8)</a> processes. Its purpose is to accept connections from the network and to decide what SMTP clients are allowed to talk to Postfix. According to the 2008 MessageLabs annual report, 81% of all email was spam, and 90% of that was sent by botnets; by 2010, those numbers were 92% and 95%, respectively. While <a href="postscreen.8.html">postscreen(8)</a> keeps the zombies away, more <a href="smtpd.8.html">smtpd(8)</a> processes remain available for legitimate clients. </p> <p> The <a href="postscreen.8.html">postscreen(8)</a> server is available with Postfix 2.8 and later. To keep the implementation simple, <a href="postscreen.8.html">postscreen(8)</a> delegates DNS white/backlist lookups to <a href="dnsblog.8.html">dnsblog(8)</a> server processes, and delegates TLS encryption/decryption to <a href="tlsproxy.8.html">tlsproxy(8)</a> server processes. This delegation is invisible to the remote SMTP client, and is not shown in the diagram below. </p> <table> <tr> <td> zombie </td> </tr> <tr> <td> </td> <td align="left"> <tt> \ </tt> </td> </tr> <tr> <td> zombie </td> <td align="left"> <tt> - </tt> </td> <td> </td> <td> </td> <td> </td> <td align="right"> <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a> </td> </tr> <tr> <td> </td> <td align="right"> <tt> \ </tt> </td> <td> </td> <td align="left"> <tt> / </tt> </td> </tr> <tr> <td bgcolor="#f0f0ff" align="center"> other </td> <td> <tt> --- </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="postscreen.8.html">postscreen(8)</a> </td> </tr> <tr> <td> </td> <td align="right"> <tt> / </tt> </td> <td> </td> <td align="right"> <tt> \ </tt> </td> </tr> <tr> <td bgcolor="#f0f0ff" align="center"> other </td> <td align="left"> <tt> - </tt> </td> <td> </td> <td> </td> <td> </td> <td align="right"> <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a> </td> </tr> <tr> <td> </td> <td align="left"> <tt> / </tt> </td> </tr> <tr> <td> zombie </td> </tr> </table> </ul> <h2> <a name="commands"> Postfix support commands </a> </h2> <p> The Postfix architecture overview ends with a summary of command-line utilities for day-to-day use of the Postfix mail system. Besides the Sendmail-compatible <a href="sendmail.1.html">sendmail(1)</a>, <a href="mailq.1.html">mailq(1)</a>, and <a href="newaliases.1.html">newaliases(1)</a> commands, the Postfix system comes with it own collection of command-line utilities. For consistency, these are all named post<i>something</i>. </p> <ul> <li> <p> The <a href="postfix.1.html">postfix(1)</a> command controls the operation of the mail system. It is the interface for starting, stopping, and restarting the mail system, as well as for some other administrative operations. This command is reserved to the super-user. </p> <li> <p> The <a href="postalias.1.html">postalias(1)</a> command maintains Postfix <a href="aliases.5.html">aliases(5)</a> type databases. This is the program that does the work for the <a href="newaliases.1.html">newaliases(1)</a> command. </p> <li> <p> The <a href="postcat.1.html">postcat(1)</a> command displays the contents of Postfix queue files. This is a limited, preliminary utility. This program is likely to be superseded by something more powerful that can also edit Postfix queue files. </p> <li> <p> The <a href="postconf.1.html">postconf(1)</a> command displays or updates Postfix <a href="postconf.5.html">main.cf</a> parameters and displays system dependent information about the supported file locking methods, and the supported types of lookup tables. </p> <li> <p> The <a href="postdrop.1.html">postdrop(1)</a> command is the mail posting utility that is run by the Postfix <a href="sendmail.1.html">sendmail(1)</a> command in order to deposit mail into the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> directory. </p> <li> <p> The <a href="postkick.1.html">postkick(1)</a> command makes some Postfix internal communication channels available for use in, for example, shell scripts. </p> <li> <p> The <a href="postlock.1.html">postlock(1)</a> command provides Postfix-compatible mailbox locking for use in, for example, shell scripts. </p> <li> <p> The <a href="postlog.1.html">postlog(1)</a> command provides Postfix-compatible logging for shell scripts. </p> <li> <p> The <a href="postmap.1.html">postmap(1)</a> command maintains Postfix lookup tables such as <a href="canonical.5.html">canonical(5)</a>, <a href="virtual.5.html">virtual(5)</a> and others. It is a cousin of the UNIX makemap command. </p> <li> <p> The <a href="postmulti.1.html">postmulti(1)</a> command repeats the "postfix start" etc. command for each Postfix instance, and supports creation, deletion etc. of Postfix instances. For a tutorial, see <a href="MULTI_INSTANCE_README.html">MULTI_INSTANCE_README</a>. </p> <li> <p> The <a href="postqueue.1.html">postqueue(1)</a> command is the privileged command that is run by Postfix <a href="sendmail.1.html">sendmail(1)</a> and <a href="mailq.1.html">mailq(1)</a> in order to flush or list the mail queue. </p> <li> <p> The <a href="postsuper.1.html">postsuper(1)</a> command maintains the Postfix queue. It removes old temporary files, and moves queue files into the right directory after a change in the hashing depth of queue directories. This command is run at mail system startup time and when Postfix is restarted. </p> </ul> </body> </html>